1. Field of the Invention
This invention relates to optical fiber processing. More specifically, the invention is a method of eliminating crystals in non-oxide optical fiber preforms and optical fibers drawn therefrom.
2. Description of the Related Art
Signal propagation through traditional silica-based optical fibers is limited by the narrow band of wavelengths they can efficiently transmit. Intrinsic losses due to Rayleigh scattering and infrared absorption limit the effective transmission band of silica to visible and near-infrared wavelengths, with an absolute minimum signal attenuation of 0.16-dB/km at a transmission wavelength of 1.55 μm. In contrast, non-oxide glasses (e.g., chalcogenide glasses and heavy metal fluoride glasses such as “ZBLAN” (ZrF4—BaF2—LaF3—AIF3—NaF)) are highly transparent from near ultraviolet to mid-infrared wavelengths. For example, ZBLAN has predicted minimum attenuation coefficients approaching 0.001-dB/km at 2.0 μm. The power of a transmitted signal decreases exponentially as a function of the attenuation of the coefficient and fiber length, hence a 100-fold reduction in the attenuation coefficient has a tremendous impact on both signal quality and propagation distance. The ability to transmit wideband signals through a low-loss fiber over significant distances has substantial commercial and national interest.
Non-oxide glasses are difficult to draw into fibers and tend to crystallize upon cooling. Crystallization causes internal reflection and refraction of the transmitted signal, thereby increasing the attenuation coefficient resulting in significantly degraded fiber performance. Because of crystallization, the best transmission obtained to date with ZBLAN optical fibers is only about 20% of the performance obtained with standard silica-fibers. Performance problems are caused by the low viscosity of the fluoride glasses at the temperatures required to melt and draw the optical fibers. Viscous shear and/or gravity-driven convection cells form within the molten glass fiber during the drawing process, creating crystal structures within the fiber.